The reliability of systems, such as computer systems, is impacted when defects exist in components of the system. In particular, if circuit lines located within a computer system have defects such as constrictions, thinned regions or cracks, the system may operate unpredictably. Therefore, techniques for detecting defects in circuit lines and other conductive patterns are continuously being sought. Such techniques include, for example, using a computer to analyze optical images in order to detect and locate visually obvious defects on exposed circuit lines; taking direct measurements of line resistance in order to detect defects that are large enough to significantly affect the total line resistance; and detecting non-linear resistance effects due to localized heating at a partial electrical blockage to detect defects in exposed or buried conductors.
Described below are a couple of examples of techniques for detecting defects. For example, in U.S. Pat. No. 4,072,895, entitled "Eddy Current Converter for Non-Destructive Testing of Electrically Conducting Coating in Holes of Printed Circuit Boards," issued on Feb. 7, 1978, to Rogachev et al. a technique for detecting defects is described. The eddy current converter described in U.S. Pat. No. 4,072,895 is used for non-destructive testing of electrically conductive coating in holes of printed circuit boards and not of circuit lines themselves. The eddy current converter comprises a cylindrical frame enveloped by an exciting winding and a measuring winding. The coils of the exciting and measuring windings extend along the generatrices of the cylindrical frame. The converter is inserted into a hole of a printed circuit board being tested.
In another example, a method and apparatus for detecting flaws in ferromagnetic materials, such as alloys containing iron, cobalt or nickel, are described in U.S. Pat. No. 5,008,620 entitled, "Leakage Flux Flaw Detection Method and Apparatus Utilizing a Layered Detector," issued on Apr. 16, 1991, and assigned to Agency of Industrial Science and Technology; Ministry of International Trade and Industry.
The leakage flux flaw detection technique described in U.S. Pat. No. 5,008,620 comprises applying one surface of a magnetic recording material to the test surface of a test material, overlaying the other surface of the magnetic recording material with a layer of magnetic material with the intervention of a non-magnetic layer and forming a magnetic path that passes through the test material. The magnetic path facilitates the flow of the leakage flux, increases the magnetization of the magnetic recording material and enables flaws to be detected.
The previously known techniques for detecting defects typically involve the detection of small defects within large metal areas and not the detection of conductive pattern defects in fine line geometry. Many of the techniques require electrical contact to the device being tested and only provide point probing information rather than image information.
Therefore, a need exists for a method and apparatus for providing a fine line resistance image of a conductive pattern. Further, a need exists for a mechanism for detecting defects, which does not require electrical contact, and applies to non-magnetic conductors of fine line geometry. Further, a need exists for a mechanism which provides image information rather than point probing information. Yet another need exists for a mechanism which provides for the imaging of lines using a low inductance head structure that is capable of producing high field strengths at a high frequency.